A diverse range of wireless communication systems are in use today enabling communication between two or more entities, such mobile stations or other user equipment. Examples of wireless communications systems include, without limiting to these, GSM (Global System for Mobile Communication), EDGE (Enhanced Data Rate for GSM Evolution), GPRS (General Packet Radio Services), and so called 3G (Third Generation) systems such as CDMA (Code Division Multiple Access) and WCDMA (Wideband CDMA). These systems are examples of radio access technologies (RAT).
Presently, GSM, a so called 2G (second generation) system, is widely used by operators of wireless networks. However, wireless systems have been developing at a rapid pace and more advanced 3G systems, such as WCDMA, are predicted to supersede 2G systems in the next few years. Network operators therefore have to consider how to migrate from one system, such as GSM or EDGE, to another, such as WCDMA, smoothly and cost effectively. One solution proposed is for an operator to maintain their existing GSM/EDGE system whilst introducing a WCDMA network that can run concurrently, and that can ultimately be merged seamlessly together with it, forming a ‘multiradio’ network.
FIG. 1 shows generally the architecture for a network operating under WCDMA. Such a network is sometimes referred to a Universal Mobile Telecommunications System (UMTS). The network comprises a number of base-stations (BSs) 101, 102 and 103. Each base-station has a radio transceiver capable of transmitting radio signals to and receiving radio signals from the area of cells 104, 105 and 106. By means of these signals the base-station can communicate with a mobile station (MS) 107. Each base station is linked to a single radio network controller (RNC) 108. An RNC can be linked to one or more BSs. An RNC can be linked to another RNC via an Iur interface 120. Each RNC is linked by an Iu interface 121 to a core network (CN) 109. The CN includes one or more serving nodes that can provide communication services to a connected mobile station, for example a mobile switching centre (MSC) 110 or a serving GPRS (general packet radio service) support node (SGSN) 111. These units are connected by the Iu interface to the RNCs. The CN is also connected to other telecommunications networks such as a fixed line network, PSTN (public switched telephone network), 114, the Internet 115 and another mobile network 116 to allow onward connection of communications outside the UMTS network. The BSs and the RNC and their interconnections constitute a UMTS terrestrial radio access network (UTRAN).
When the mobile station (MS) moves between cells during a communications connection there is a need to hand it over from communication with the BS of the old cell to the BS of the new cell without dropping the call due to a break in communications between the mobile station and the network. This process is known as handover. A need can also arise to hand over the MS even when it does not move, for example when local conditions affect its communications in the old cell and call quality can be improved by handing over to another cell or if there is a need to free up capacity in the old cell, e.g., due to overloading.
Handovers may also occur in other systems such as GSM or EDGE. The reasons for handover may also be similar, though it will be appreciated by one skilled in the art that the elements of the network may be different to those shown in FIG. 1 if the system differs.
In both proposed WCDMA systems and existing GSM/EDGE systems, the coverage area of the network may be made up various cell types such as macrocell, microcell and picocell. Macrocells may be defined as cells having the largest coverage area, followed by microcells, with picocells defined as cells having the smallest coverage area. It should be appreciated that areas containing a large number of MSs, such as in cities, would generally be served by microcells or picocells, as a collection of several microcells or picocells would be able to handle more MSs and traffic than a macrocell with the same coverage area. Conversely, areas with lower MS numbers would generally be served by macrocells. However, the types of cells used in any given area are not mutually exclusive, and typically a layered approach may be adopted. This is where macrocells, microcells and picocells are all used within the same area in an overlapping/layered manner. The result is a more robust network that is better equipped to handle traffic fluctuations as handovers can be more effectively used to share traffic load across the overlapping layers of cells and not just to extend coverage across adjacent cells.
Such a layered approach is particularly important in a multiradio network, which may incorporate, for example, both GSM/EDGE and WCDMA. FIG. 2 shows a layered structured of cells in an example of part of a multiradio network cell structure 400. The network comprises a macrocell layer 250 of a macrocell 201 operating under GSM/EGDE and a macrocell 202 operating under WCDMA at frequency f1. With substantially the same coverage area as the macrocell layer is a microcell layer 260. The microcell layer 260 comprises microcells 203, 204 and 205 operating under GSM, and microcells 206, 207 and 208 operating under WCDMA at frequency f2. Another layer, a picocell layer 270, operates below the microcell layer. The picocell layer 270 comprises picocells 209 and 210 operating under WCDMA at frequency f3, 211 and 212 operating under TDD (Time Division Duplex access mode), and 213, 214, 215 and 216 operating under a WLAN (wireless local area network) system. This layered architecture is particularly suited to traffic load sharing described earlier.
It should be appreciated that the number and coverage area of each type of cell may vary, and is not limited to those depicted in FIG. 2. For example, the macrocells may have the same coverage area as 4 or 5 microcells instead of the 3 depicted.
In a multiradio network, it is important to utilize all the systems (e.g. GSM, WCDMA) or carriers (e.g., different layers of WCDMA operating at different frequencies) in the most efficient way possible. For example, the coverage area of the network can be increased by moving users from one cell to an adjacent cell, such as between GSM microcells 203 and 204, or WCDMA picocells 209 and 210 by utilizing handover techniques.
The capacity of the network can also be increased by moving users from a highly loaded cell to one with a low load by utilizing traffic reason handover techniques.
Service changes may also trigger a handover. Typical services may include real time services such as conversational communications (at various data rates), data streaming (at various data rates), and non real time services such as interactive web browsing and emailing. A handover may take place when a neighboring cell/system is better suited to provide the service requested.
Traffic load handover may be triggered when the load of a cell increases beyond a certain threshold, and the overloaded cell instructs one or more of the MSs in the cell to perform handovers.
In general, these handovers fall into two types: inter-frequency/carrier (IF) and inter-system (IS) or inter-RAT (Radio Access Technology). IF handover takes place when the systems of the cells stay the same, but the frequency of the systems change. This can occur for example between WCDMA systems operating in different frequency bands. For example, a handover from WCDMA macrocell 202 to WCDMA microcell 207 would constitute an IF handover. IS handover is handover across systems. For example, a handover from WCDMA microcell 207 to GSM macrocell 201, or from WLAN picocell 213 to WCDMA picocell 209 would constitute IS handovers.
Present radio access networks have consisted of just a single radio access network, such as GSM, or several independent systems. Some functionality is available for load sharing and interference distribution within GSM networks, but the efficiency of such methods are limited to the area controlled by the associated radio resource controller, such as or a Base Station Controller (BSC) in GSM system or an RNC in the WCDMA system illustrated in FIG. 1. As such, selection of the new cell/system in handovers in a multiradio system has been less than adequate. This is partly due to the limitations in the signaling between the different controllers. Thus, load information sharing is not generally available between cells, and selection of a new cell/system is based only on received signal strength or quality and some predefined offsets and parameters. These may include, for example, minimum signal level thresholds for target system/cell to be selected or some offset for target system/cell's signal strength over the current level in the source cell.
To this end, Common Resource Radio Management (CRRM) and Common Radio Management Server (CRMS) have been introduced to help manage some of these issues that have arisen in multiradio systems. Their roles include overall resource management of controller and system borders to provide load sharing for efficient use of resources, interference distribution to provide higher spectral efficiency and improved QoS (quality of service) management. CRRM can be implemented in both a centralised in a CRMS and distributed manner across other elements such as RNCs.
FIG. 3 illustrates the structure of a typical distributed CRRM arrangement. Here the CRRM entities, 304, 306 and 308, are located at each of the RNCs/BSCs 301, 302 and 303. Radio resource management within each controller is managed by the Radio Resource Management entity (RRM), 305, 307 and 309. The CRRMs are responsible for control between the RRM entities, and communication between the controllers is done via the CRRMs.
FIG. 4 illustrates the structure of a centralised CRRM arrangement utilizing a CRMS. The arrangement comprises RNC/BSC 403, 405 and 406, associated with RRMs 404, 406 and 408 respectively. Control of these RRMs is done by a centralized CRRM entity, the CRMS 401.
When a MS is connected to a GSM cell, the transmission gaps in GSM communications can be used for continuous measurements of other systems or layers to assist in handovers. These measurements may include determining cell ID (identity) information required for connecting to a cell of new system/frequency. However, when a MS is in connected to a WCDMA cell its scope for making measurements at other frequencies or of other systems is significantly reduced because of the continuous transmission nature of communications in WCDMA. Therefore, any measurements in WCDMA have to be typically performed in compressed mode.
In compressed mode, reception of a signal is stopped for a certain period of time to enable the MS to measure at another frequency. To achieve this, the data has to be compressed before sending it to the MS. This data compression is controlled by the RNC.
However, in compressed mode, data is transmitted at a higher power, generating more interference, which affects cell capacity further. Therefore, measurements in compressed mode are generally kept to a minimum. Compressed mode measurements take time, which delays the handover procedure. Simultaneous IF and IS compressed mode measurements might not be allowed due to practical limitations such as time limitations on measurement of multiple neighbor cells. Furthermore the number of GSM BSICs (base station identification codes) that can be decoded from neighboring cells is limited. All these factors impose practical limitations on the number of target cells that can be measured before handover takes place.
As both IF and IS compressed mode measurements made simultaneously might not be possible, the selection between IF and IS compressed mode measurements must be made after handover is triggered. If the selection is based on only signal strength and/or service priorities then the undesirable selection of a highly loaded cell is possible. Furthermore, as CM measurements are time consuming, a reduction in such measurements is desirable to reduce handover delays. Selecting the correct system is also important. For example, if the call is a circuit switched speech service then preference may be for the cell to be a GSM microcell. Conversely, if the call is packet switched conversational service (e.g., video telephony) then a WCDMA cell may be preferred.
Another problem arises when a MS is connected to a WCDMA cell and is making IS compressed mode measurements to GSM cells. IS compressed mode measurements generally take two parts: a received signal strength indicator (RSSI) measurement from all neighboring GSM cells; and BSIC decoding for all (RSSI) measured neighboring GSM cells. BSIC decoding is very time consuming, and typically, the BSIC of the measured GSM neighbor with the highest RSSI is decoded and chosen as the target cell.
This can significantly reduce CRRM performance since a highly loaded GSM cell could be selected as a target cell even if there were lower loaded cells with adequate signal levels available.
When setting the threshold over which handover should be triggered in a source cell, factors such as potential overlap of the neighboring cells as well as the neighboring cells' loads should be considered. Handover should only be triggered if there are neighboring cells available to handover to with a lower load, hence any threshold that is set for a source cell must take this into account. Neighboring cells that are not overlapping or at adjacent to the source cell should not influence the setting of the threshold as much. However, when handover is to be made intersystem, then information relating to the precise coverage area of target cells may not be readily available, thus making calculation of a handover threshold for the source cell difficult when presented only with a list of neighboring cells and their associated loads. The problem is not just limited to intersystem handovers, and similar problems arise with inter-frequency handovers and trying to estimate cell loads and handover thresholds.